This invention relates to a medical antenna catheter apparatus and a method for measuring the contact force applied by an antenna catheter to tissue during a medical procedure.
Our technique may be used any time it is desireable to avoid placing too much force on a medical catheter or the tissue contacted thereby, such as when steering a catheter along a passage in the body or when stabilizing the catheter at a particular location in the body. It is especially advantageous for use during a cardiac ablation procedure. Accordingly, the invention will be described in that context.
Antenna catheters or probes utilize electromagnetic radiation to simultaneously controllably heat, and detect the temperature of, tissue contacted by a catheter or probe. By placing the probe at the region of interest in the body, in this case a target site on the heart, one can treat certain abnormalities, such as a cardiac arrhythmia for example.
Obviously, in order to perform its function, such a probe must be small in diameter and quite flexible and steerable so that it can be threaded into the body to the target site via a natural passage in the body. It may also be required to facilitate various ancillary processes such as display of the target site, cooling or irrigation of the target site, etc.
Antenna probes of the above type are invariably connected by a long cable to an external control unit which includes a transmitter for transmitting electromagnetic energy to the antenna in the probe in order to heat tissue adjacent to the probe, and a receiver in the form of a radiometer which can detect microwave emissions picked up by the probe antenna which emissions reflect the temperature at depth of the tissue adjacent to the probe.
The receiver produces corresponding output signals to control a display which displays the tissue temperature. Those same signals may also be used to control the transmitter to maintain a selected heating profile.
During a typical cardiac ablation procedure, the antenna catheter or probe is used to resistively heat heart tissue usually at the left side of the heart sufficiently to intentionally damage the tissue in order to cure a potentially fatal heart arrhythmia. Typically, heating the tissue to a temperature in excess of 70° C. for 30-60 seconds is sufficient to cause necrosis. This procedure was first attempted over twenty years ago and has become the standard treatment method for most supraventricular tacchycardias (SVTs). It may also be used to treat paroxysmal atrial fibrillation (AF). During treatment, electromagnetic energy, usually in the RF frequency range, is applied between the tip of the antenna probe and a ground plate removably affixed to the patient's back, creating an electrical circuit. The point of highest resistance in this circuit, normally at the interface between the probe tip and the heart tissue, is the region which heats the most and thus may cause irreversible damage to the heart tissue.
In a standard SVT procedure, the heat generated in the tissue contacted by the probe is monitored with a temperature sensor such as a thermister or a thermocouple in the probe tip. A signal from the sensor is applied to a display in the external unit, enabling the operating surgeon to adjust the power as needed to provide sufficient heating of the tissue to cause necrosis, but not enough to result in surface charring of the tissue that could cause a stroke and/or the formation of bubbles (popping) that could rupture the heart vessel wall. The same output from the temperature sensor is sometimes used to provide a feedback signal to the transmitter to achieve controlled heating of the tissue contacted by the probe.
When placing a conventional probe in the heart, it is recognized that the operating surgeon should not press the probe too hard against the tissue. It is fairly well established in the medical literature that the load on the tissue due to the probe should not exceed 0.050 newtons (N), the equivalent of 50 grams of weight. The problem is that the surgeon may not be able to appreciate how much contact force he/she is actually applying to the tissue. This is especially the case if the surgeon is using a robotic system to position the probe and thus has little or no tactile feedback. Indeed, there have been reported cases where the operating physician or robot places too much force on the tissue, causing the probe tip to perforate the heart wall. This condition, known as cardiac perforation, can lead to serious complications including death of the patient.
Also, with experience over time, surgeons have found a need to burn tissue on the left side of the heart increasingly deeper to achieve a favorable patient outcome. In order to minimize the above-mentioned surface charring of the tissue, the tips of today's antenna probes may be cooled by circulating a fluid through the probe. However, with this artificial cooling came the undesired consequence that the attending surgeon no longer has the temperature feedback described above because the temperature sensor in the probe no longer takes accurate tissue temperature readings at the point of contact. That is, since the probe tip is being cooled, the temperature sensor in the tip measures the tip temperature, not the tissue temperature.
Another disadvantage of the prior apparatus of this type is the operating surgeon has to work within the radiation field of the antenna catheter during the entire ablation procedure. Such exposure over the years could cause irreparable harm to the surgeon. This may be avoided using robotics allowing for the remote placement of the probe at the target site. However, until now it has proven difficult to control a robotic arm with sufficient accuracy to enable placement of an antenna catheter against the heart tissue with just the right amount of contact force to enable tissue ablation at the point of contact without damaging the heart as described above.
One can envision other medical procedures, e.g. angioplasty, colonoscopy, etc., wherein the movements of a catheter in the body could cause an excessive contact force to be applied to the opposing portions of the tissue and probe resulting in damage to one or both of same.